The present invention relates to a countermeasure against cold-time sounds in a rolling contact bearing or a pulley having a rolling contact bearing built therein.
For example, a tension pulley frequently used in a belt transmission for automobiles is of the type in which, as shown in FIG. 4, a pulley body 11 and a ball bearing 12 are integrated together for weight reduction. The pulley body 11 is a press-molded steel plate part, comprising an outer cylindrical portion 11a adapted to have a belt (not shown) entrained thereabout, and an inner cylindrical portion 11b in which the outer ring 12b of the ball bearing 12 is fitted. The ball bearing 12 is of the deep groove type, comprising an inner ring 12a to be fitted on a shaft (not shown), the outer ring 12b fitted in the inner cylindrical portion 11b of the pulley body 11, a plurality of balls 12c interposed between the raceway tracks of the inner ring 12a and the outer rings 12b, a cage 12d of synthetic resin for holding the balls 12c at circumferentially equispaced intervals, and seals 12e for sealing grease. When this tension pulley is rotated with the pulley body receiving a rotative power from the belt, the outer ring 12b of the ball bearing 12 fitted therein is rotated in a unit with the pulley body 11.
In this connection, when such tension pulley is operated at a cold time, it sometimes produces abnormal sounds. The cause of such abnormal sounds at a cold time or the so-called cold-time sounds still remains to be proved precisely. Furthermore, since tension pulleys used in automobiles and the like are operated under high temperature, high speed and high load conditions so that its durability is also an important characteristic, countermeasures which lead to a decrease in durability cannot be taken. For such reason, at present no decisive countermeasure has been developed against cold-time sounds occurring in tension pulleys.
It has heretofore been studied to use a grease which is superior in low temperature properties (such a grease as will uniformly form oil films in the areas of contact between the rolling elements and the raceway surfaces of the inner and outer rings). This countermeasure, which is intended to suppress the occurrence of cold-time sounds, can be expected to provide substantial effects. However, since the viscosity of the grease is lowered, there is a fear that the lubricating performance at high temperatures is degraded, leading to a decrease in durability.
Further, there are reports saying that occurrence of cold-time sounds has been suppressed by increasing the radius of curvature of the raceway surfaces of the inner and outer rings or by increasing the bearing clearances; however, such increase in the radius of curvature of the raceway surface or in bearing clearances results in angular swaying of the pulley body, tending to spoil the function of the tension pulley.
Further, there is a design in which in order to suppress the vibration sound (cage sound) due to a play in the cage, the cage is contacted with the outer or inner ring with a negative clearance defined therebetween. With this arrangement, however, the wear in the areas of contact between the cage and the inner and outer rings is liable to decrease the durability. Furthermore, the cage is always rotated at a speed greater than the speed of planetary rotation of the balls (because the cage is rotated as it is restrained by the inner or outer ring), resulting in pushing the balls hard at all times in the direction of planetary rotation of the bails; thus, the normal rotation of the balls is impeded, so that on the contrary, cold-time sounds are increased or even if they are not increased, the effect of suppressing them cannot be expected.